1. Field of the Invention
The present invention relates to a falling body viscometer for measuring a falling speed of a falling body which is caused to fall into a substance to be measured in a measuring container, and for calculating a viscosity of the measured substance based on a measured value. More particularly, the present invention relates to a technique for enhancing accuracy in a measurement of a falling speed.
2. Background
Conventionally, there is known a viscometer for causing a falling body to fall into a liquid which is to be a measuring target for a viscosity, and for calculating the viscosity of the liquid based on a falling speed of the falling body (for example, see Patent Documents 1 and 2, identified below).
In the Patent Document 1, there is employed a structure in which an almost needle-like falling body is caused to fall into a tubular measuring container, and particularly, it is used for measuring a viscosity of blood. More specifically, there is employed a structure in which a pair of electromagnetic induction sensors separated from each other vertically is attached to the tubular measuring container, there is measured a time taken for receiving a detection signal of the almost needle-like falling body through the lower electromagnetic induction sensor after receiving a detection signal of the almost needle-like falling body through the upper electromagnetic induction sensor, and a falling termination speed is detected from the time and a distance between the upper and lower electromagnetic induction sensors. The “falling termination speed” is set to be a falling speed in uniform falling in a fluid.
Moreover, the Patent Document 1 also describes the configuration for measuring a falling termination speed of an almost needle-like falling body by using a pair of electromagnetic induction sensors, and furthermore, measuring a falling acceleration of the almost needle-like falling body by using three electromagnetic induction sensors.
There is not disclosed a specific structure of the electromagnetic induction sensor described in the Patent Document 1. For example, a magnetic field is applied to a coil surrounding the measuring container, a change in a voltage based on an induced electromotive force generated in a passage of the almost needle-like falling body is determined, and a time that the voltage is maximized is defined as a time that the coil and the almost needle-like falling body (a metallic weight) are the closest to each other. Consequently, it is possible to define a time that the almost needle-like falling body passes through a position of the electromagnetic induction sensor (a coil surface).
As shown in FIG. 8, however, an actual signal output (V) contains a noise e caused by a fluctuation in a current value for applying a magnetic field or the like, and a change rate of the signal output is minimized in the vicinity of a maximum value as is illustrated in an example of a result of a measurement in which an axis of abscissa indicates a time (t) and an axis of ordinate indicates a signal output (V) of an induced electromotive force. Therefore, a value of the signal output (V) is apt to be influenced by the noise e. In other words, the value of the signal output (V) in the vicinity of the maximum value greatly reflects the noise e. For this reason, it is hard to accurately define a time T1 (time) corresponding to a position in which the signal output (V) excluding the noise e is maximized (a position for defining a distance between the upper and lower electromagnetic induction sensors), that is, a true position S1.
The axis of abscissa in a graph shown in FIG. 8 also corresponds to upper and lower positions of the almost needle-like falling body, and also represents that the signal output (V) of the induced electromotive force is gradually increased when the fall progresses so that the almost needle-like falling body approaches the position S1 with a passage of the time, and is gradually decreased apart from the position S1.
For instance, in the example of FIG. 8, the time T1 originally corresponds to the true position S1, and the output signal is maximized by the noise e at a time T2. If a position S2 corresponding to the time T2 is defined as a position (time) in which the signal output (V) is maximized, therefore, the almost needle-like falling body is regarded to pass through the electromagnetic induction sensor at the time T2 corresponding to the position S2 which is shifted from the true position S1.
In the method described above, thus, it is hard to accurately define the time that the almost needle-like falling body passes through the position of the electromagnetic induction sensor (the coil surface) in consideration of the noise e, and the time that the almost needle-like falling body passes through the position of the electromagnetic induction sensor is defined in a state in which the influence of the noise e is directly reflected with reference to only the position S2 (time T2) on a point specified as described above. Consequently, the time that the almost needle-like falling body passes through the position of the electromagnetic induction sensor is defined with a variation depending on a situation of the noise e so that a falling speed of a falling body to be measured has a variation. There is a fear that accuracy in a measurement of a viscosity obtained based on the falling speed might not be excellent.
In the case in which a measurement for a viscosity of blood is assumed, particularly, a quantity of the blood to be taken for measuring the viscosity is limited. Therefore, it is an important object to enhance accuracy in the measurement.